It is known:
(a) to treat a surface of a cup-shaped container by bringing a suitable fluid into contact with such surface;
(b) to electro-phoretically coat a surface of a metal, cup-shaped container by bringing an electro-coating liquid into contact with the surface and simultaneously applying an electric potential between that surface and an adjacent electrode;
(c) to electro-phoretically coat an internal surface only of a metal cup-shaped container by means of a static body of electro-coating liquid disposed between the upright container (open and uppermost) and an internal electrode which has been lowered into a predetermined charge of such liquid present in the container until the displaced liquid contacts the whole of the interior surface to be coated (Patent specification GB 455,810, Crosse & Blackwell Ltd., published 1936);
(d) to electro-phoretically coat an interior surface only of an un-enclosed, inverted, metal cup-shaped container by means of a flowing electro-coating liquid which is brought into contact with that surface by causing it to issue upwards from an internal electrode nozzle on to the interior surface of the base portion of the inverted container and to flow thence downwardly in the space between the interior surface of the container side wall and internal electrode U.S. Pat. No. 3,922,213, Aluminium Company of America, published 1975);
(e) to treat (e.g. clean in preparation for filling) a cup-shaped container both internally and externally in the same operation by enclosing the container in an upright position in a cell with small clearance between the surfaces of the container and the respective adjacent surfaces of the cell structure, the cell having in one embodiment a loose, hinged and apertured base member (or lid) to enable the container to be introduced from below, and in the principal embodiment two vertically slidable and engageable cell body halves to enable the container to be introduced horizontally from the side, the treating operation being carried out solely by means of a high speed turbulent fluid flowing downwardly to the internal base surface and thence in succession over both of the internal and external surfaces of the container to exit through the aperture in the cell base member, and without any electrical energisation of such fluid (U.S. Pat. No. 3,969,136, Oxy Metal Industries Corporation, published 1976);
(f) to treat (e.g. clean in preparation for filling) a cup-shaped container both internally and externally in the same operation by enclosing the container in an inverted position in a cell with small clearance between the surfaces of the container and the respective adjacent surfaces of the cell structure; which structure comprises a cell body into which the inverted container is drawn by suction, and a loose lid which is removable vertically and angularly, the lid being disposed at the upper end or the lower end of the cell body, and the treatment being carried out by flowing the treatment liquid successively over both of the internal and the external surfaces of the container (in either direction) in such manner as to produce turbulence and so enhance the cleaning action of the liquid, but without any electrical energisation of the liquid (Patent specification GB 1,498,795, Jackson, published 1978);
(g) to electro-phoretically coat a cup-shaped container both internally and externally in the same operation by enclosing the container in an inverted position in a cell with small clearance between the surfaces of the container and the respective adjacent (electrode) surfaces of the cell structure, which structure comprises vertically separable cell body and cell lid portions, the lid portion being lowermost and receiving on it the open end of the inverted container, and the electro-coating process being carried on by flowing the coating liquid successively over both of the internal and the external surfaces of the container in either direction (U.S. Pat. No. 4,094,760, Aluminium Company of America, published 1978, and corresponding GB 2,001,348, published 1979);
(h) to electro-phoretically coat a cup-shaped container both internally and externally in the same operation by enclosing the container in an inverted position in a cell with small clearance between the surfaces of the container and the respective adjacent (electrode) surfaces of the cell structure, which structure comprises a cell comprising vertically separable cell body and cell lid portions, the lid portion being uppermost and closing the upper open end of the cell body portion, and the electro-coating being carried on by flowing the coating liquid successively over both of the external and the internal surfaces of the container (Patent specification GB 1,604,035, Metal Box plc, published 1981); and
(i) to electro-phoretically coat the internal surface and the external base surface of a cup-shaped container by enclosing the container in the inverted position in a cell with small clearance between those internal and external surfaces and their respective adjacent (electrode) surfaces of the cell structure, which structure comprises vertically separable cell body and cell lid portions, the lid portion being uppermost and closing the upper open end of the cell body portion, and the electro-coating being carried on by simultaneously flowing the coating liquid in parallel over the internal surface and the external base surface of the container (Patent specification GB 2,085,474 B, Metal Box plc, published 1982).
In the upright container arrangement referred to at (c) above (GB 455,810--electro-coating the internal surface only) and at (e) above (U.S. Pat. No. 3,969,136--treating non-electrically both internal and external surfaces), the cup-shaped container of (c) above has to be turned upside down to remove the charge of electro-coating fluid, whilst in the arrangement of (e) above, the treatment fluids are removed by the subsequent high speed passage of high pressure purging air.
In the arrangement referred to at (h) above (GB 1,604,035), at the high liquid flows required to fill an empty the inter-cell and container passages in the time available, small variations in the flow rate which occurred at some parts of the flow passages led to the formation of voids in the coating liquid, with the result that some areas of the surfaces to be coated were left uncoated. Furthermore, the use of high speed compressed air to expel the coating liquid from those passages after coating had been completed and to eject the container from the cell body has led to excessive foaming of the coating liquid circulating in the cell (and could lead to an excessive build-up of foam in the coating liquid circulation system), and to the presence of foam on some surface parts of the ejected container which foam is difficult to remove during subsequent rinsing and which can therefore give rise to surface defects in the film produced by thermally curing the electro-coating material.
The present invention seeks to provide a method of electro-coating a cup-shaped container, which method (1) avoids the difficulties experienced with that prior art electro-coating apparatus and (ii) enables (a) the electro-coating of can bodies at very high through-puts, e.g. of the order of six hundred cans per minute, (b) the production of electro-coated can bodies which are consistently and uniformly coated to a very high standard for all through-put rates within the design range of the apparatus, and (c) the production of can bodies at a through-put rate which is variable to suit the conditions prevailing in the can body production and processing line of which the electro-coating apparatus forms part. The invention also seeks to provide an apparatus for enabling the improved electro-coating method of the present invention to be successfully carried out.